The asynchronous transfer mode (ATM) based on an asynchronous time-division multiplex technique plays an important part in recent developments of telecommunications technology and in the development of integrated services broadband networks (B-ISDN). In this mode the signal transmission ensues in a bit stream which is subdivided into cells. Each cell, composed of a header part and a useful information part, has a constant length of, for example, 53 octets that are occupied as needed with packeted messages. When useful information is not to be communicated at the moment, then specific dummy cells are transmitted. Virtual connections are set up in ATM switching centers. The virtual connections are connections that actually only use a route section when a message packet (block) is actually to be communicated thereover. The header of each packet contains, among other things, an address covering, for example, two octets for the unambiguous allocation of the packet to a specific virtual connection. In accordance with the respective dial information, each packet can thereby receive the complete information for its route through the switching network at the input to the switching network. The switching elements then through-connect the packet on the defined route themselves (self-routing network; see, for example, Telcom Report 11 (1988) 6, 210 . . . 213).
When one forgoes a header translation or implements this in electronic devices, switching networks for ATM message cells can also be realized with optically transparent devices for queuing and routing functions.
For example, European reference EP-A2-0 313 389 (corresponding to U.S. Pat. No. 4,894,818) discloses an optical packet switching system having optical 2.times.2 coupling switches arranged in switching stages. Every coupling switch has buffer memory means at its two inputs that lead to the input of an optical switch-over means whose two outputs form the two coupling switch outputs. The optical switch-over means is preceded in the light waveguide path by an optical demultiplexer with which only light having a wavelength defined individually per switching stage can be coupled out from a routing header. Proceeding from this demultiplexer, the optical switch-over means following in the light waveguide path is controlled via an optoelectric transducer. The optical switch-over means enters into one or the other of its switch states dependent on whether or not the wavelength defined for the appertaining switching stage is contained in the routing header.
In order to avoid cell losses, it is thereby known to provide cell memories that are composed of a fed back line of the switching network having a delay of at least one cell duration. In this known light waveguide telecommunication system, both routing functions as well as associated queuing functions connected therewith in order to avoid cell losses are optically implemented. However, the queuing functions are limited to the insertion of message packets of two input lines onto a common, continuing line.
The same queuing principle can also be employed for a switching matrix wherein the switching matrix outputs are connected to switching matrix inputs via light waveguide delay lines having graduated transit times equal to the message cell duration or to a multiple thereof.
For example, a single-stage or multi-stage ATM switching network is known wherein respectively two successive, optical space switching multiples are connected to one another by light waveguides having a negligibly short transit time as well as by optical intermediate memories. Message cells can then be through-connected across the optical switching network undelayed or with different delays as needed. The optical intermediate storage thereby occurs in an optical apparatus, hereinafter referred to as an optical transit time harp, consisting of a plurality of light waveguides having graduated transit times whose transit times are equal to whole-number n-multiples of the message cell duration in order to avoid cell losses given occupied switching network outputs (see European reference EP-92108243.4). Additionally or alternatively, an optical switching network can have a part of the outputs of an optical space-switching multiple connected to a corresponding plurality of inputs of the space-switching multiple via an optical transit time harp formed with a plurality of light waveguides having graduated transit times. As a result message cells can be through-connected across the optical switching network practically undelayed or with different delays as needed. The transit time harp can have light waveguides having graduated transit times that are shorter than the duration of a message cell in order to reduce a more or less pronounced jitter of message cells that occur unsynchronized (see German reference DE-A1-4 216 077).